1. Field of the Invention
The invention relates to an apparatus for fabricating an optical fiber and a method for sealing a drawing furnace used in a drawing of the optical fiber, which is suitable for drawing a optical fiber preform with no grip portion having a constant diameter.
2. Description of the Related Art
An optical fiber is fabricated by heating and melting a distal end of a silica glass rod so-called an optical fiber preform (hereinafter simply referred as a preform) while moving downwardly, and drawing an optical fiber from the melted distal end of the preform. The interior of a drawing furnace when fabricating the optical fiber reaches a temperature of about 2000 degrees C., i.e. immensely-high, so that carbon is generally used as a material for elements such as a furnace core tube as a heater. To prevent the carbon from erosion by oxidization, it is required to maintain the interior of the drawing furnace with an inert gas atmosphere such gases as argon, helium or the like, and control external air flow into the interior of the drawing furnace.
In a conventional way, to control external air flowing into the drawing furnace, a gas sealing ring with an inner diameter corresponding to an outer diameter of the preform is arranged at a preform insertion opening so as to provide a small clearance gap between the preform insertion opening and the preform. On the other hand, if a fluctuation of the outer diameter of the preform is bigger than some extent, at the top end of the drawing furnace, the clearance gap between the preform and the gas sealing ring can be significantly changed in size in response to the fluctuation of the outer diameter of the preform when drawing the optical fiber, depending on the position of the preform. The fluctuation of the clearance gap in size causes a change of the flow of the inert gas supplied into the drawing furnace, so that it is impossible to control the outer diameter of the optical fiber. Additionally, if the size of the clearance gap becomes larger than a predetermined value, external air flows into the interior of the drawing furnace will erode the carbon element(s) by oxidization. As a result, dust from an eroded portion adheres to a melted portion of the glass preform 1. The adhered dust causes a localized fluctuation of the outer diameter and localized embrittlement of the optical fiber 5, as well as a decrease in the life of the carbon part of the drawing furnace, so that the optical characteristic and the strength of the optical fiber might degrade.
To avoid the above problem, a preform with a finely adjusted outer diameter is generally used. To obtain the preform having a finely adjusted outer diameter, first, a deposition process, such as a Vapor-phase Axial Deposition (VAD) process or an Outside Vapor Deposition (OVD) process, is conducted to produce a porous glass preform. Next, the porous glass preform is heated so as to sinter the preform and produce a transparent vitrified glass preform with a relatively large fluctuation in its outer diameter in the longitudinal direction. Finally, the transparently vitrified glass preform is heated and drawn so as to achieve the preform with a smaller and a finely adjusted outer diameter.
To change an entire preform to an optical fiber, for example, a dummy rod or a dummy tube with the same outer diameter as that of the preform is connected to the proximal end of the preform. The dummy-portion is supported by a feed mechanism for feeding the preform into a drawing furnace. As a result, it is possible to boost yield in fabricating an optical fiber by heating the entire preform from its distal end to its proximal end at a central area of the drawing furnace so as to make an optical fiber.
However, if conducting the above discussed heating and drawing process for finely adjusting the outer diameter of the preform or using the dummy rod or dummy tube increases the cost. To avoid the additional cost, it is required to draw an optical fiber directly from a preform which is not subject to a drawing process and has fluctuation in its outer diameter.
FIG. 1 schematically illustrates a structure of a preform produced by the VAD process or the OVD process. The preform 1 has a constant diameter portion 1a, tapered portions 2 and 3 with a length of from about 10 mm to 500 mm at both end portions, and a core 6. The core 6 is in the constant diameter portion 1a, but is not in the tapered portions 2 and 3. The outer diameter of the constant diameter portion 1a is typically from 10 mm to 200 mm and has an outer diameter fluctuation of from several millimeters to several tens of millimeters. The tapered portions 2 and 3 have gripped portions 4 and 5 at their ends, respectively. The diameter of the gripped portions 4 and 5 is typically from 20 mm to 70 mm. The gripped portions 4 has a projection 7 engaged with a transfer jig when transferring the preform 1.
To change the entire preform 1 to an optical fiber, external air must be prevented from flowing into the drawing furnace when the tapered portion 2 at the proximal side of the preform 1 passes through an insertion opening of the drawing furnace.
However, a clearance gap formed between the insertion opening and the tapered portion 2 is too big so that it is difficult to efficiently seal the clearance gap with only a gas sealing ring arranged at the insertion opening. An inefficient seal of the clearance gap causes external air to flow into the drawing furnace when making an optical fiber so that the optical characteristic or the strength of the optical fiber might degrade.
For example, Japanese Patent Laid-open No. 2006-342030 (corresponding to US 2006/280578A1) discloses a method capable of sealing a gap between an insertion opening of a drawing furnace and a preform in response to a fluctuation in the outer diameter of the preform. The method comprises pushing a sealing ring, formed of a combination of an inner ring and an outer ring respectively configured with a plurality of mutually connected seal ring fragments, onto a surface of a glass preform. Elastic springs are arranged on a periphery of the outer ring.
In this method, however, a sealable fluctuation range of the outer diameter, which the sealing ring can efficiently seal, is limited to a predetermined value. As a result, efficient seal can not be provided with regard to a preform with a tapered portion having a wide fluctuation range of its outer diameter.
Japanese Patent Laid-open No. 2007-70189 discloses a sealing member which is provided with a brush and arranged at an insertion opening. However, an effective sealable fluctuation range of the outer diameter of a preform is also limited to a predetermined value, so that an efficient seal can not be provided with regard to a preform with a tapered portion having a wide fluctuation range of its outer diameter.
A method for drawing an optical fiber from the foregoing preform is known in which the entire of the preform is covered by a sealed chamber (a top chamber) disposed on a top end of a drawing furnace. However, in this method, to move the preform hung in the top chamber into the drawing furnace, a very long shaft for supporting the preform is needed and the overall height of the apparatus is very long.
To avoid those problems, Japanese Patent Laid-open No. 63-176330 discloses a top chamber configured with bellows which can elongate and contract.
However, because the bellows are repeatedly extended and contracted under a high temperature, they might break or generate dust. Additionally, a heater which can reach a temperature of over 2000 degrees C. is arranged below the top chamber, so that a strong convection is generated in the top chamber and a gas flow is disturbed in the vicinity of the melted of the preform. As a result, a large fluctuation of the outer diameter of the drawn optical fiber might be generated.